High-speed I/O (Input/Output) circuits used in servers and the like for an optical communication network are communication interface circuits for data transmission between processors included in the servers or between the servers.
The electrical characteristics of each element included in I/O circuits having such uses are controlled according to variation in the characteristics of circuit elements, a system environment, or the like in order to realize normal high-speed transmission.
Formerly the technique of a programmable cell array in which output from each register is inputted to a programmable multiplexer and in which a signal to be outputted from the multiplexer is determined by the state of each fuse was proposed.
Furthermore, the following technique was proposed. A control unit provides data to a PLL circuit by giving a control signal to an electric fuse array, and exercises control so as to make the PLL circuit operate on the basis of a specific combination of characteristics.
Japanese Laid-open Patent Publication No. 05-218199
Japanese Laid-open Patent Publication No. 2007-110711
An I/O circuit includes registers which are storage elements for control. By setting a determined data value in a register, the electrical characteristics of an element in the I/O circuit are controlled. A data value may be set in a register by software. In this case, however, setting time becomes longer with an increase in the number of registers. Accordingly, a data value may be set in a register by hardware.
However, if an initial data value set in the beginning is not most suitable, then a change in hardware, such as mask revision or mask re-creation, is made to set a data value again. This leads to a decrease in the efficiency of development.